As early as the sixth century before the Christian era, and while their knowledge of Northern Europe was still very imperfect, the Greeks had already become aware of the existence of the British Isles. This comparatively early knowledge of Britain was derived from the trade in tin, for which there existed at that period an extensive demand in the East. It was imported by sea by the Phœnicians, and by their colony, the Carthaginians, who extended their voyages beyond the Pillars of Hercules; and was subsequently prosecuted as a land trade by their commercial rivals, the Greek colonists of Marseilles. A Greek poet, writing under the name of Orpheus, but whose real date may be fixed at the sixth century, mentions these remote islands under the name of the Iernian Isles;[19] but in the subsequent century they were known to Herodotus as the Cassiterides, or Tin Islands,[20] a name derived from the chief article of the trade through which all report of their existence was as yet derived. In the fourth century they are alluded to by Aristotle as two very large islands beyond the Pillars of Hercules; and, while the name of Britannia was now from henceforth applied, especially by the Greek writers, to the group of islands, of whose number and size but vague notions were still entertained, the two principal islands appear for the first time under the distinctive appellations of Albion and Ierne.[21] Polybius, in the second century before Christ, likewise alludes to the Britannic Islands beyond the Pillars of Hercules, and to the working of the mines by the inhabitants.[22] Besides these direct allusions to the British Isles, we have preserved to us by subsequent writers an account of these islands from each of the two sources of information—the Phœnician voyages and the land trade of the Phocæans of Marseilles—in the narratives of the expeditions of Himilco and Pytheas. Himilco was a Carthaginian who was engaged in the Phœnician maritime trade in the sixth century, and the traditionary account of his voyage is preserved by a comparatively late writer, Festus Rufus Avienus. In his poetical Description of the World, written from the account of Himilco, he mentions the plains of the Britons and the distant Thule, and talks of the sacred isle peopled by the nation of the Hiberni and the adjacent island of the Albiones.[23] Pytheas was a Massilian. His account of his journey is preserved by the geographer Strabo, and appears to have been received with great distrust. He stated that he had sailed round Spain and the half of Britain; ascertained that the latter was an island; made a voyage of six days to the island of Thule, and then returned. From him Strabo, Diodorus Siculus, and Pliny derived their information as to the size of the islands, and his statement made known for the first time the names of three promontories—Cantium or Kent, Belerium or Land’s End, and Orcas, or that opposite the Orkneys.[24]